1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device including a transparent conductive layer and a fabricating method thereof.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices are commonly used for flat panel displays (FPDs) because they are lightweight and consume relatively low amounts of power. However, LCD devices are not light-emitting displays. As such, LCDs have several disadvantages including dim displays, poor contrast ratios, narrow viewing angles and small display sizes. Accordingly, new FPDs, such as organic electroluminescent (EL) devices, have been developed to solve these problems. Organic EL devices are light-emitting displays that possess a wider viewing angle and a better contrast ratio than LCD devices. Furthermore, since no backlight is required for an organic EL device, organic EL devices generally are both lighter and thinner than LCD devices, and consume less power. Organic EL devices may be driven with a low direct current (DC) voltage that permits a faster response speed than LCD devices. Moreover, since organic EL devices are solid-phase devices, unlike LCD devices, they can better withstand external impacts and possess a greater operational temperature range. In addition, organic EL devices may be manufactured more cheaply than LCD devices or plasma display devices (PDPs) because organic EL devices require only deposition and encapsulation apparatus.
A passive matrix design that does not use additional thin film transistors (TFTs) may be used for organic EL devices. However, passive matrix organic EL devices have limited display resolution, relatively high power consumption, and a relatively short expected life span. Thus, active matrix organic EL devices have been developed as next-generation display devices that provide high resolution over a large display area. In passive matrix organic EL devices, a scan line and a signal line cross each other to provide a switching element for a sub-pixel. In contrast, a TFT, disposed at each sub-pixel, is used as a switching element in active matrix organic EL devices. The TFT is used to turn each sub-pixel ON or OFF. Specifically, a first electrode, which is connected to the TFT, is turned ON or OFF by the sub-pixel, and a second electrode, which faces the first electrode, functions as a common electrode.
FIG. 1 is an energy band diagram of an organic electroluminescent device according to the related art. In FIG. 1, the organic electroluminescent device includes an anode 1 and a cathode 7 that are separated from each other, a hole injection layer 2, a hole transporting layer 3, an emission layer 4, an electron transporting layer 5, and an electron injection layer 6 interposed between the anode 1 and the cathode 7. The hole injection layer 2 and the electron injection layer 6 contact the anode 1 and the cathode 7, respectively. A hole that is on the anode 1 passes through the hole injection layer 2 and the hole transporting layer 3 and is injected into the emission layer 4. An electron that is on the cathode 7 passes through the electron injection layer 6 and the electron transporting layer 5 and is injected into the emission layer 4. The hole and the electron injected into the emission layer 4 form an exciton 8, and light is emitted by the formation of the exciton 8. Since mobility of the hole and mobility of the electron are substantially different in an organic material, the hole and the electron are effectively transported to the emission layer by using the hole transporting layer and the electron transporting layer. Thus, the multi-layer organic electroluminescent device has a high emission efficiency due to a balance of densities of holes and electrons in the emission layer.
The anode 1 may be made of a transparent conductive material such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The cathode 7 may be made of a chemically stable material whose work function is lower than that of the anode 1. As the work function of the cathode is lowered, a lower driving voltage is required. Moreover, a lower work function results in improved brightness and current density. The cathode may be made of aluminum (Al), calcium (Ca), lithium:aluminum (Li:Al), or magnesium:silver (Mg:Ag).
In FIG. 1, since an anode 1 made of ITO has a work function between about 4.7 eV and about 4.8 eV, and the hole injection layer 2 has a work function between about 5.2 eV and about 5.3 eV, an energy band gap “I” exists between the anode 1 and the hole injection layer 2. Accordingly, the injection efficiency of a hole from the anode 1 is reduced due to the work function difference between the anode 1 and the hole injection layer 2. Since an organic electroluminescent device uses carrier injection, the device performance is reduced by the loss of injection efficiency of the carrier. Moreover, adhesion between the anode and the organic thin film is basically poor in the organic electroluminescent device. As a result, the anode and the organic thin film may separate due to differences in their interface voltages and thermal expansion coefficients at high driving voltages or high temperatures. Accordingly, degradation of the organic electroluminescent device may occur, which may result in a reduction in the expected life span of the organic electroluminescent device.